Evolution Can Be Skin Deep: Biological Evolution in Humans
“We see beautiful adaptation everywhere and in every part of the organic world.”
-- Charles Darwin, Origin of Species
According to Darwin and his theory on evolution, organisms are presented with nature’s challenge of environmental change. Those that possess the characteristics of adapting to such challenges are successful in leaving their genes behind and ensuring that their lineage will continue. It is natural selection, where nature can perform tiny to mass sporadic experiments on its organisms, and the results can be interesting from extinction to significant changes within a species.
Human beings are no exception to biological evolution. Like other organisms around the world, humans have significantly changed overtime and have developed all sorts of diverse characteristics. One noticeable characteristic of human beings is the variation of skin color. Skin color has been used to identify, classify, and verify the variation that exists in the human population around the world. How did such a distinct variation arise and how did it play into adaptation?
I’ve often heard that “humans came from monkeys,” or something similar. It is true that humans’ ancestors were primates, who first resided in warm and sunny Africa; they had similar features to today’s apes, such as a hairy body. The purpose of the vast amount of hair was to protect the body from the sun’s ultraviolet (UV) rays and to prevent overheating, mainly by acting as a barrier for the skin underneath the hair (Jablonski, 598). Some parts of the body such as palms were not covered with hair, but with sweat glands. Sweat glands allowed the body to cool off via evaporation at the surface of the skin; sweat glands were more efficient at thermoregulation. Overtime, early humans with a high amount of sweat glands were selected for since they had the best method at the time to keep themselves cool in warm environments (Kirchweger). This meant that overtime, humans lost most of their hair on their bodies, leaving their skin exposed. Sweat glands were going to help the body to cool down, but they couldn’t protect the skin from harmful UV rays. This is where melanin worked its magic, and it’s the reason for the diversity in skin color today.
Melanin helps reduce the absorption of wavelengths into the skin (Chaplin, Jablonski, 59). The more melanin in the skin, the greater the protection against harmful UV rays, and the amount of melanin in the skin correlates with the skin’s color (more melanin means darker skin). Because Africa was a very warm climate that received lots of sunlight, humans went from having thick fur-like hair to almost hairless, dark skin in order to protect them from detrimental effects of UV rays. However, like humans today, the ancestors of modern humans did not stay in one place and various groups moved to various regions of the world. Moving to new regions meant coping and adapting to a new environment.
Geographical isolation in the past has caused a variation in skin color; natural selection selected specific skin pigments depending on environmental factors. Early humans who moved to Europe were introduced to a place that had less sunlight than their former place of residence. Overtime, light skin (skin with lower levels of melanin) was selected for since there were not as many UV rays that the skin needed protection from. But, what was wrong with having dark skin in Europe? How is extra melanin a disadvantage in this case? One thing to remember is that even though the sun can provide UV rays, sunlight is also a great source of natural vitamin D (Jablonski; Kirchweger). Vitamin D helps bones absorb calcium and is particularly essential for developing embryos in pregnant women. The adaptation for lighter skin was important because the skin needed to absorb as much sunlight as it could in order to receive optimal amounts of vitamin D (Kirchweger). However, too much vitamin D can be fatal. So as high amounts of melanin protected the skin from excessive UV rays in Africa, they also protected it from excessive vitamin D (Kirchweger).
Lighter skinned humans had an advantage with obtaining an optimal amount of vitamin D because their skin drank in the sunlight. Darker skinned humans had an advantage over another nutrient: folic acid (folate). Folate is essential for proper DNA synthesis. The lack of substancial folate can lead to disorders in developing embryos and infertility in males (Chaplin, Jablonski 61). UV rays contribute to folate photolysis (the breakdown of folate), therefore a high melanin content in the skin can cut down on such damage. Overall, both light skin and dark skin have their advantages when it comes to living in certain environments and taking in certain nutrients. Yet, how does skin color play a role in our modern world today, and how will it affect the future?
In today’s world, due to extraordinary technological advancements, the opportunity for people to move when and where they want is possible. Take a look at the United States, a country made of people moving in. There are people from all over the world, with all hues of skin colors. With interracial dating and marriages and more “mixing” of skin colors, there are multiracial children with various skin colors. Genetically, it enhances variation within the individual, but what about variation within the species? I heard a joke once that eventually in the future, everyone will be beige. Will everyone mix together to an extent that there will be little to no variation anymore, at least skin deep? It’s an interesting concept to think of. At the moment, I believe that there is enough diversity within the human species that we don’t need to worry about the lack of variation in the near future. For now, we can appreciate the diversity of skin colors that has allowed our ancestors to adapt to their environments and survive. It has allowed them to create a lineage of who we are today.
1) Chaplin, G. Jablonski, N. “The Evolution of Human Skin Coloration.” Journal of Human Evolution 39 (2000) 57-106
2) Jablonski N. “The Evolution of Human Skin and Skin Color” Annual Reviews Anthropology 33 (2004) 585-623
3) Kirchweger G. “The Biology of… Skin Color” Discover 22 (2001)